U.S. patent application number 15/075535 was filed with the patent office on 2016-09-29 for real current meter.
The applicant listed for this patent is Liebert Corporation. Invention is credited to Charles F. BLAIR, Terry D. BUSH, James K. MARTIN.
Application Number | 20160282390 15/075535 |
Document ID | / |
Family ID | 56976272 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160282390 |
Kind Code |
A1 |
MARTIN; James K. ; et
al. |
September 29, 2016 |
Real Current Meter
Abstract
A real current meter reads current from a current probe coupled
around power lines of a transformer-based UPS system coupled to a
transformer having a high resistance ground with a HRG resistance
and determines a real current component of the current read from
the current probe.
Inventors: |
MARTIN; James K.; (Galena,
OH) ; BUSH; Terry D.; (Westerville, OH) ;
BLAIR; Charles F.; (Powell, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Liebert Corporation |
Columbus |
OH |
US |
|
|
Family ID: |
56976272 |
Appl. No.: |
15/075535 |
Filed: |
March 21, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62139252 |
Mar 27, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 31/52 20200101;
G01R 31/50 20200101; G01R 19/02 20130101; G01R 19/06 20130101 |
International
Class: |
G01R 19/06 20060101
G01R019/06; G01R 19/02 20060101 G01R019/02 |
Claims
1. A real current meter, comprising: a current sensor coupled to a
controller, the current sensor couplable to a current probe that is
couplable around three phases of power lines of a transformer-based
uninterruptible power supply system, the three phases being phase
.phi..sub.a, phase .phi..sub.b which lags phase .phi..sub.a by 120
degrees and phase .phi..sub.c which lags phase .phi..sub.a by 240
degrees; a voltage sensor coupled to the controller, the voltage
sensor couplable across two of the phases; the controller
configured to determine a real current component of current sensed
via the current sensor when the current probe is coupled around the
three phases and one of the phases is experiencing a ground fault
and the voltage sensor is coupled across the two phases by:
determining a resultant angle based on time elapsed between a zero
cross time of an RMS current sum sensed via the current sensor and
a zero cross time across the phases to which the voltage sensor is
coupled; setting a fault angle to the resultant angle when the
resultant angle is between zero and ninety degrees, setting the
fault angle to the resultant angle decremented by one-hundred
twenty degrees when decrementing the resultant angle by one-hundred
twenty degrees results in an angle between zero and ninety degrees
and setting the fault angle to the result angle decremented by
two-hundred forty degrees when decrementing the resultant angle by
two-hundred forty degrees results in an angle between zero and
ninety degrees; determining the real current component of the RMS
current sum by multiplying the RMS current sum by a cosine of the
set fault angle; and the controller configured to display on a
display the determined real current component of the RMS current
sum.
2. The real current meter of claim 1 wherein the controller is
configured to determine the resultant angle by converting the time
elapsed between the zero cross time of the RMS current sum and the
zero cross time across the phases to which the voltage sensor is
coupled to an angle and adding thirty degrees to this angle.
3. The real current meter of claim 2 wherein when the voltage
sensor is coupled across phase .phi..sub.a and .phi..sub.b the
controller is configured to determine that the ground fault is on
phase .phi..sub.a when the resultant angle is between zero and
ninety degrees, configured to determine that the ground fault is on
phase .phi..sub.b when the resultant angle is between 120 degrees
and 210 degrees and configured to determine that the ground fault
is on phase .phi..sub.c when the resultant angle is between 240
degrees and 330 degrees.
4. A method of measuring a real current component of an RMS current
sum of current flowing in three phases of power lines for a three
phase transformer-based uninterruptible power supply system having
a transformer having a high resistance ground connection wherein
one of the three phases is experiencing a ground fault, the three
phases being phase .phi..sub.a, phase .phi..sub.b which lags phase
.phi..sub.a by 120 degrees and phase .phi..sub.c which lags phase
.phi..sub.a by 240 degrees, the uninterruptible power supply system
having switched circuits that include capacitors, the method
comprising: placing a current probe coupled to a current sensor of
a real current meter around the three phases; coupling a voltage
sensor of the real current meter across phase .phi..sub.a and phase
.phi..sub.b; with a controller of the real current meter:
determining a resultant angle based on time elapsed between a zero
cross time of the RMS current sum sensed via the current sensor and
a zero cross time across the phases a and b and with the
controller: setting a fault angle to the resultant angle when the
resultant angle is between zero and ninety degrees; setting the
fault angle to the resultant angle decremented by one-hundred
twenty degrees when decrementing the resultant angle by one-hundred
twenty degrees results in an angle between zero and ninety degrees;
and setting the fault angle to the resultant angle decremented by
two-hundred forty degrees when decrementing the resultant angle by
two-hundred forty degrees results in an angle between zero and
ninety degrees; determining the real current component of the RMS
current sum by multiplying the RMS current sum by a cosine of the
set fault angle; and displaying on a display of the real current
meter the determined real current component of the RMS current
sum.
5. The method of claim 4 wherein determining the resultant angle
with the controller includes having the controller determine the
resultant angle by converting the time elapsed between the zero
cross time of the RMS current sum and the zero cross time across
the phases to which the voltage sensor is coupled to an angle and
adding thirty degrees to this angle.
6. The method of claim 5 including determining with the controller
which of the phases the ground fault is on by determining that the
ground fault is on phase .phi..sub.a when the resultant angle is
between zero and ninety degrees, determining that the ground fault
is on phase .phi..sub.b when the resultant angle is between 120
degrees and 210 degrees and determining that the ground fault is on
phase .phi..sub.c when the resultant angle is between 240 degrees
and 330 degrees.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/139,252, filed on Mar. 27, 2015. The entire
disclosure of the above application is incorporated herein by
reference.
FIELD
[0002] The present disclosure relates to meters used in diagnosing
faults in uninterruptible power supply systems.
BACKGROUND
[0003] This section provides background information related to the
present disclosure which is not necessarily prior art.
[0004] In transformer-based uninterruptible power supply systems
(UPS systems) such as UPS system 100 (FIG. 1) having a rectifier
input 101 coupled to an input transformer 102 with a Y secondary
104, the Y secondary 104 often has a high resistance ground
connection, often referred to as an HRG. That is, as shown in FIG.
1, Y secondary has an HRG 106 with a resistance referred to herein
as HRG resistance 108 connected between the common connection 110
of the Y secondary 104 and actual ground. The hot phases Y-1, Y-2,
Y-3 of Y secondary 104 are coupled to input 101 of UPS system 100
and provide the AC power input to UPS system 100.
[0005] When there is no ground fault in the UPS system 100, the 3
phase currents in power lines 112, 114, 116 from the three hot
phases Y-1, Y-2, Y-3 of Y secondary 104 will sum to zero. Thus, a
current probe 118 coupled around power lines 112, 114, 116 will see
zero current.
[0006] When there is a ground fault, shown as ground fault 120 in
FIG. 1, on one of power lines 112, 114 or 116, the current flowing
through the HRG resistance 108 is measured as part of the process
of diagnosing the short. In the example shown in FIG. 1, a meter
122 is coupled to current probe 118 to read the current sensed by
current probe 118. When current probe 118, which is coupled around
power lines 112, 114, 116, is located between Y-secondary 104 and
ground fault 120, current probe 118 will see only the HRG current
(as the three phase currents sum to zero) which will be read by
meter 122 which for example displays the current it reads. When
current probe 118 is located between ground fault 120 and UPS
system 100, current probe 118 does not see the HRG current and thus
the current read by meter 122 from current probe 118 will be
zero.
[0007] To locate ground fault 120, current probe 118 is positioned
at different locations along power lines 112, 114, 116 typically by
starting adjacent Y-secondary 104 and moving current probe 118
along power lines 112, 114, 116 until the current seen by current
probe 118 goes to zero. This occurs when current probe 118 is moved
across power lines 112, 114, 116 where ground fault 120 has
occurred. It should be understood that current probe 118 could be
moved along power lines 112, 114, 116 in the opposite direction and
the location of ground fault 120 then identified when the current
seen by current probe 118 goes from zero to the HRG current. In
some cases, the resistance value of the HRG resistance 108 can be
switched making it easier to locate the fault as the meter readings
change as the HRG resistance value changes and the changing meter
readings are more noticeable.
[0008] The foregoing is also applicable to a transformer-based UPS
system having an inverter output coupled to a Y-primary of an
output transformer which has an HRG. It should be understood that
the UPS system have both an n input transformer an output
transformer having a Y-primary, with the input of the rectifier of
the UPS system coupled to the Y-secondary of the input transformer
and the output of the inverter of the UPS system coupled to the
Y-secondary of the output transformer, with both the input
transformer and the output transformer having an HRG. The hot
phases of the Y-secondary of the input transformer and of the
Y-primary of the output transformer will be referred to generically
herein as phases .phi..sub.a, .phi..sub.b, .phi..sub.c. The voltage
on phase .phi..sub.b lags the voltage on phase .phi..sub.a by 120
degrees and the voltage on phase .phi..sub.c lags the voltage on
phase .phi..sub.a a by 240 degrees.
[0009] The foregoing discussion assumed that there is only real
current flowing in phases .phi..sub.a, .phi..sub.b, .phi..sub.c. In
modern UPS systems, this is not the case as the capacitors in the
switched circuits of the UPS systems, the EMI capacitors in
particular of the EMI filters of the UPS systems, have an
appreciable amount of reactive current flowing through them,
typically referred to as charging current.
[0010] Consequently, current probe 118 will also see this non-zero
charging current in the power lines for phases .phi..sub.a,
.phi..sub.b, .phi..sub.c which will then be read by meter 122. As a
result, while current probe 118 will see a change in the amount of
current as it is moved across the location of ground fault 120,
this change will not be between zero and the amount of the HRG
current. Rather, the change will be between the charging current
and the sum of the charging current and the HRG current.
SUMMARY
[0011] This section provides a general summary of the disclosure,
and is not a comprehensive disclosure of its full scope or all of
its features.
[0012] In accordance with an aspect of the present disclosure, a
real current meter reads current from a current probe coupled
around power lines of a transformer-based UPS system coupled to a
transformer having a high resistance ground with a HRG resistance
and determines a real current component of the current read from
the current probe.
[0013] In an aspect, the real current meter has a current sensor
coupled to a controller, the current sensor couplable to a current
probe that is couplable around three phases of power lines of the
uninterruptible power supply system, the three phases being phases
.phi..sub.a, .phi..sub.b, .phi..sub.c with an AC voltage on phase
.phi..sub.b lagging an AC voltage on phase .phi..sub.a by 120
degrees and an AC voltage on phase .phi..sub.c lagging the AC
voltage on phase .phi..sub.a by 240 degrees. The real current meter
has a voltage sensor coupled to the controller, the voltage sensor
couplable across two phases of the input power lines. The
controller is configured to determine a real current component of
current sensed via the current sensor when the current probe is
coupled around the three phases of the power lines and one of the
phases is experiencing a ground fault and the voltage sensor is
coupled to two phases of the power lines by: determining a
resultant angle based on time elapsed between a zero cross time of
an RMS current sum sensed via the current sensor and a zero cross
time across the phases of the input lines to which the voltage
sensor is coupled; setting a fault angle to the resultant angle
when the resultant angle is between zero and ninety degrees,
setting the fault angle to the resultant angle decremented by
one-hundred twenty degrees when decrementing the resultant angle by
one-hundred twenty degrees results in an angle between zero and
ninety degrees and setting the fault angle to the result angle
decremented by two-hundred forty degrees when decrementing the
resultant angle by two-hundred forty degrees results in an angle
between zero and ninety degrees and determining the real current
component of the RMS current sum by multiplying the RMS current sum
by a cosine of the set fault angle. The controller is also
configured to display on a display the determined real current
component of the RMS current sum.
[0014] In an aspect, the controller is configured to determine the
resultant angle by converting the time elapsed between the zero
cross time of the RMS current sum and the zero cross time across
the phases of the input lines to which the voltage sensor is
coupled to an angle and adding thirty degrees to this angle.
[0015] In an aspect, when the voltage sensor is coupled across
phase .phi..sub.a and phase .phi..sub.b he controller is configured
to determine that the ground fault is on phase .phi..sub.a when the
resultant angle is between zero and ninety degrees, configured to
determine that the ground fault is on phase .phi..sub.b when the
resultant angle is between 120 degrees and 210 degrees and
configured to determine that the ground fault is on phase
.phi..sub.c when the resultant angle is between 240 degrees and 330
degrees.
[0016] Further areas of applicability will become apparent from the
description provided herein. The description and specific examples
in this summary are intended for purposes of illustration only and
are not intended to limit the scope of the present disclosure.
DRAWINGS
[0017] The drawings described herein are for illustrative purposes
only of selected embodiments and not all possible implementations,
and are not intended to limit the scope of the present
disclosure.
[0018] FIG. 1 is a simplified schematic of a prior art UPS
system;
[0019] FIG. 2 is a simplified schematic showing a real current
meter in accordance with an aspect of the present disclosure
coupled to power lines between a Y-secondary of an input
transformer having a HRG resistance and a rectifier input of a
prior art UPS system;
[0020] FIG. 3 is a simplified schematic showing the real current
meter of FIG. 2 coupled to power lines between an inverter output
of a prior art UPS system and a Y-primary of an output transformer
having an HRG resistance; and
[0021] FIG. 4 is a flow chart of a control routine used in the real
current meter of FIGS. 2 and 3 to determine HRG current.
[0022] Corresponding reference numerals indicate corresponding
parts throughout the several views of the drawings.
DETAILED DESCRIPTION
[0023] Example embodiments will now be described more fully with
reference to the accompanying drawings. Corresponding reference
numerals indicate corresponding parts throughout the several views
of the drawings.
[0024] FIG. 2 shows a real current meter 200 in accordance with an
aspect of the present disclosure having a current sensor 202
coupled to current probe 118 that is coupled around input power
lines 112, 114, 116 (phases .phi..sub.a, .phi..sub.b .phi..sub.c)
from Y-secondary 104 of input transformer 102 to a rectifier input
203 of UPS system 204. UPS system 204 is shown illustratively with
EMI filtering capacitors 206 as representative of capacitors in the
switched circuits of UPS system 204 associated with rectifier input
203 of UPS system 204. Real current meter 200 also includes a
voltage sensor 208 coupled to two of .phi..sub.a, .phi..sub.b
.phi..sub.c of input power lines 112, 114, 116 and a controller 210
that determines real current based on readings from current probe
118 and voltage sensor 208, as discussed in more detail below.
Current sensor 202 and voltage sensor 208 are coupled to controller
210. The real current determined by controller 210, shown
representatively by box 212, is optionally filtered by low pass
filter 214 and the filtered real current output to display 216
where a value of the real current is displayed. It should be
understood that optional low pass filter 214 could be implemented
in controller 210, in software for example, and controller 210 then
drives display 216 to display the value of the real current.
[0025] FIG. 3 shows real current meter 200 having current sensor
202 coupled to current probe 118 that is coupled around output
power lines 300, 302, 304 (phases .phi..sub.a, .phi..sub.b
.phi..sub.c respectively) from an inverter output 316 of a UPS
system 308 to a Y-primary 310 of an output transformer 312. UPS
system 308 is shown illustratively with EMI filtering capacitors
314 as representative of capacitors in the switched circuits of UPS
system 308 associated with inverter output 316 of UPS system
308.
[0026] The following methodology is illustratively used in
determining HRG current. HRG current is the result of current
flowing through the HRG resistor when a ground fault occurs and
will be in phase with the voltage of the phase on which the ground
fault is present, referred to herein as the faulted phase. The
charging current is the result of reactive current flowing through
the EMI capacitors and will lead the faulted phase voltage by
ninety degrees. The net sum of these two currents (HRG current plus
charging current) will flow at some angle between zero and ninety
degrees in relation to the faulted phase voltage. The angle will
depend on the relative amplitudes of the HRG current and the
charging current. It should be understood that the HRG current is
real current and thus the real current component of the net sum of
these two current components.
[0027] If the RMS current sum (I.sub.sum) is measured (which is the
net sum of the HRG current and charging current) and the angle
between it and the faulted voltage phase determined, referred to
herein as the fault angle, then the HRG current and the charging
current can each be derived. This is done by multiplying the RMS
current sum by the cosine of this fault angle to get the HRG
current and by multiplying the RMS current sum by the sine of this
fault angle to get the charging current. The charging current is
thus separated from the RMS current sum to get the desired HRG
current.
[0028] The fault angle between the RMS current sum and the faulted
phase voltage (V.sub.a, V.sub.b or V.sub.c) is determined in
accordance with the following. The time elapsed between I.sub.sum
zero cross time and the zero cross time of the voltage across
phases .phi..sub.a, .phi..sub.b (V.sub.ab) is measured. This time
is converted to an angle by multiplying it by the frequency of the
system (50 or 60 Hz) and then multiplying the result by 360
degrees. Since the voltage of phase .phi..sub.a to neutral
(V.sub.an) lags V.sub.ab by thirty degrees, thirty degrees is added
to this angle with the summed angle referred to as the resultant
angle. If this resultant angle is between zero and ninety degrees,
the fault is on .phi..sub.a, and the fault angle is this resultant
angle. If not, this resultant angle is reduced in steps of 120
degree decrements until the decremented angle is between zero and
ninety degrees. If one decrement of 120 degrees results in the
decremented angle being between zero and ninety degrees, the ground
fault is on .phi..sub.b and the fault angle is the resultant angle
decremented by 120 degrees. If two decrements of 120 degrees result
in the decremented angle being between zero and ninety degrees, the
ground fault is on .phi..sub.c and the fault angle is the resultant
angle decremented by 240 degrees.
[0029] FIG. 4 is a flow chart of an illustrative routine
implemented in controller 210, such as in software or firmware
programmed in controller 210, of the above methodology by which
controller 210 determines the HRG current based on the current
sensed by current sensor 202 from current probe 118 and the voltage
sensed by voltage sensor 208. As discussed above, current probe 118
is coupled around the power lines for all three phases, such as
power lines 112, 114, 116 in FIG. 2 or power lines 300, 302, 304 in
FIG. 3. Current sensor 202 senses the RMS sum, referred to as
I.sub.sum as discussed above. Voltage sensor 208 is coupled across
two of the three phases, phases .phi..sub.a and .phi..sub.b in the
following discussion, and senses the voltage across these two
phases, referred to as V.sub.ab as discussed above. At 400, the
time from I.sub.sum zero crossing to V.sub.ab zero crossing is
measured, illustratively by controller 210. At 402, the measured
time is converted to an angle, illustratively by controller 210, in
the manner discussed above, and thirty degrees is added to this
angle with this summed angle referred to as the resultant angle
also as discussed above. At 404, it is determined which phase
(.phi..sub.a, .phi..sub.b, or .phi..sub.c) the ground fault is on
based on the resultant angle, this determination illustratively
being made by controller 210. In accordance with the above
discussed methodology, if the resultant angle is between zero and
ninety degrees, the ground fault is on phase .phi..sub.a, if the
resultant angle is between 120 degrees and 210 degrees, the ground
fault is on phase .phi..sub.b and if the resultant angle is between
240 degrees and 330 degrees, the ground fault is on phase
.phi..sub.c. At 406, the fault angle is determined, illustratively
by controller 210. In accordance with the above discussed
methodology, if the resultant angle is between zero and ninety
degrees, the fault angle is set equal to the resultant angle. If
the resultant angle is between 120 degrees and 210 degrees, the
fault angle is set equal to the resultant angle minus 120 degrees.
If the resultant angle is between 240 degrees and 330 degrees, the
fault angle is set equal to the resultant angle minus 240 degrees.
It should be understood that since the current in any phase cannot
lead the voltage in that phase by more than ninety degrees, the
resultant angle can never be between ninety degrees and 120
degrees, between 210 degrees and 240 degrees, or between 330
degrees and zero degrees.
[0030] At 408, a value of HRG current is determined by multiplying
I.sub.sum by the cosine of the fault angle, illustratively by
controller 210, which is real current and equal to a real current
component of I.sub.sum. At 410, the determined value of HRG current
is output by controller 210, such as to display 216. The determined
value of HRG current may optionally be filtered by optional low
pass filter 214 to remove any higher frequency signals from the
current measurement that may result from the PWM switching of the
rectifier and/or inverter of the UPS system.
[0031] The foregoing description of the embodiments has been
provided for purposes of illustration and description. It is not
intended to be exhaustive or to limit the disclosure. Individual
elements or features of a particular embodiment are generally not
limited to that particular embodiment, but, where applicable, are
interchangeable and can be used in a selected embodiment, even if
not specifically shown or described. The same may also be varied in
many ways. Such variations are not to be regarded as a departure
from the disclosure, and all such modifications are intended to be
included within the scope of the disclosure.
* * * * *